Embryologic, Histologic, and Anatomic Aspects
The eccrine gland is the only true sweat gland in humans. Eccrine sweat is a hypotonic solution that flows from the gland to the surface of the skin where it cools the body by evaporation. Eccrine units are present nearly everywhere on human skin, including the glans penis and prepuce, but not the oral lips, clitoris, labia minora, and external auditory canals. They are most populous on the palms, soles, axillae, and forehead. Embryologically, eccrine units derive from surface ectodermis, arising independent of infundibuloapocrine-sebaceous-follicular units and descending to near the junction between the dermis and the subcutaneous fat. Some glands may be situated somewhat higher in the reticular dermis and others may be positioned well within the subcutaneous fat. Approximately 3 million eccrine sweat units are present at birth, and no additional ones are formed thereafter.
Each eccrine unit is a hollow tube bounded proximally by a gland, which is a cul-de-sac, and distally by an opening onto the skin surface. The eccrine unit can be divided into (1) a coiled secretory gland proximally, (2) a coiled dermal duct that leads from the secretory gland, (3) a straight duct that passes through the length of the dermis, and (4) a spiraled intraepidermal duct known also as acrosyringium (Fig. 1.56).
An eccrine unit is a simple hollow tube that begins in a coiled cul-de-sac deep in the reticular dermis or in the subcutaneous fat and ends on the skin surface as the terminus of a spiraled intraepidermal duct (acrosyringium). Both ends of the tube are connected by a straight duct that traverses the dermis.
The coiled gland is formed of two rows of cells (Fig. 1.57), namely, (1) a discontinuous outer row of spindle-shaped, contractile myoepithelial cells and (2) an inner row of pyramidal, secretory epithelial cells. The single inner row of secretory cells that lines the lumen of a gland consists of two types, to wit, large glycogen-containing pale or clear cells and smaller mucopolysaccharide-containing dark cells. The dark cells tend to line the luminal surface, whereas the pale cells are situated peripheral to the dark cells and, for the most part, do not abut the lumen itself. Peripheral to the outer row of myoepithelial cells is a basement membrane that separates glandular epithelium from the richly vascular connective tissue of the periadnexal dermis.
Although eccrine glands are very different morphologically from apocrine glands, eccrine ducts are identical with apocrine ducts. The secretory portion of an eccrine gland consists of two layers of cells: (1) a thin outer row of myoepithelial cells and (2) an inner row of cuboidal secretory cells. Eccrine ducts, like apocrine ducts, are lined by two layers of small cuboidal epithelial cells whose luminal edge is rimmed by homogenous eosinophilic material. (x440)
The eccrine dermal duct is lined by a double row of small, darkly basophilic, cuboidal epithelial cells (Fig. 1.57). A homogenous eosinophilic cuticle rims the luminal margin of the entire duct. Ultrastructurally, peripheral cells of a duct are replete with mitochondria, whereas luminal cells have fewer mitochondria and a dense layer of tonofilaments adjacent to their luminal membrane. Each duct enters the epidermis at the bottom of a rete ridge and widens as it spirals through the epidermis en route to opening onto the skin surface.
The eccrine ductal cells that traverse the epidermis are known as acrosyringeal cells, those that line the lumen being designated “cuticular,” and those peripheral “poroid.” As intraepidermal keratocytes of an eccrine duct near the ostium they come to contain keratohyaline granules. Throughout their course, they are linked to one another and to neighboring epidermal keratocytes by desmosomes. The keratocytes of the intraepidermal sweat duct cornify independent of epidermal keratocytes but in a manner analogous. The difference between them is witnessed best morphologically in normal skin by signs of preservation of the intraepidermal corkscrew pattern of eccrine ducts within the cornified layer of volar skin (Fig. 1.58) and, under abnormal circumstances, in pale-cell acanthoma and in solar keratosis. In each of those circumstances, the integrity of the acrosyringium may be seen to be maintained. As stated previously, the ducts of eccrine and apocrine glands, unlike the glands themselves, are indistinguishable from each other histologically. A few melanocytes are present in the upper part of an eccrine duct.
The intraepidermal portion of the eccrine duct, the acrosyringium, spirals through surface epidermis in a corkscrew pattern that is preserved throughout, even in the cornified layer as seen in this section from a palm. Keratocytes that form the acrosyringium are different morphologically and biologically from those of adjacent epidermal keratocytes. (x352)
At this juncture, a few words should be addressed to the matter of the so-called acrotrichium [Gk. acro- (highest point) + trichium (hair)], a term that was coined by Hermann Pinkus as an analogue of “acrosyringium,” a word and concept also introduced by him. In short, the term “acrotrichium” is as wrong linguistically as it is conceptually. What Pinkus called the acrotrichium is not the highest point of a hair, as the term denotes, but rather the course travelled by infundibular epidermis through surface epidermis, it having no relation to the hair follicle per se, which resides below the infundibulum, although in continuity with it. The hair itself does pass through the infundibular canal en route to reaching the surface of the skin, from which it exits by way of an ostium in the infundibulum. In short, the concept of acrotrichium is not at all analogous to that of acrosyringium, which refers specifically to the spiral of an eccrine duct through surface epidermis and to the spiral of an apocrine duct through infundibular epidermis. The notion of acrotrichium, it being flawed fatally, should be abandoned.
The axillae of adults are purported to contain another type of sweat unit termed the “apo-eccrine gland,” reputed to comprise 10% to 45% of all glands in that locale. “Apo-eccrine glands” are supposed to develop during puberty from eccrine-like precursors and are said to be larger than typical eccrine glands, but smaller than typical apocrine glands. Like stereotypical eccrine ducts, ducts of “apo-eccrine glands” are claimed to open directly onto the surface of the skin. The secretory portion of an “apo-eccrine gland” is supposed to be dilated in irregular fashion. The epithelial lining of the dilated glandular segment of an “apo-eccrine gland” is said to resemble an apocrine gland, whereas the epithelial lining of the undilated segment resembles an eccrine gland. The arguments in favor of an “apo-eccrine gland” are not compelling, and it is likely that no such gland exists.
The actual process of sweating commences in the pale (clear) cells of the secretory portion of an eccrine gland. The energy requirements for secretion of sweat are met by the numerous mitochondria and particles of glycogen present in the cytoplasm of those pale cells (Fig. 1.59). Intercellular canaliculi lined by microvilli serve as conduits for transport of solution secreted by pale cells of the gland to the lumen. Granules of dark cells secrete mucopolysaccharides that become components of the eosinophilic cuticle lining the lumen of ducts. Secretion of mucopolysaccharides is thought to facilitate subsequent reabsorption by ductal epithelium of sodium, chloride, bicarbonate, and other electrolytes. Eccrine sweat is a colorless, odorless, hypotonic solution.
A single row of secretory cells lines the lumen of an eccrine gland. It consists of (1) pale cells that contain abundant glycogen and mitochondria and (2) dark cells that have dense granules. The luminal border displays numerous villi. Myoepithelial cells surround secretory ones. (x7000) (Courtesy of Ken Hashimoto, M.D.)
Eccrine glands on the palms, soles, axillae, and forehead, unlike those on the rest of the body, respond predominantly to emotional, rather than thermal, stimuli. This produces a pattern of sweating visible easily in individuals who are experiencing pain, anger, or fear. Hyperhidrosis itself may be a source of considerable anxiety in circumstances social. Another deleterious effect of sweating occurs when allergic contact sensitizers are leached from jewelry or clothing by salts present in sweat. This enhances induction of allergic contact dermatitis in susceptible persons. Miliaria rubra, known colloquially as “heat rash” and “prickly heat,” is a spongiotic inflammatory process thought to be centered in acrosyringia and related somehow to obstruction in flow of sweat through intraepidermal ducts.